One general problem of fuel cell systems is their start-up at low temperatures, i.e., at temperatures significantly below 0° C. For example, if a fuel cell system is to be started at a temperature of −15° C., one first uses a heating device associated with the fuel cell system to heat the fuel cell system (in particular its stack of membrane-electrode assemblies) to a temperature of, for example, +5° C., and only then starts the fuel cell system. This is based on the consideration that the fuel cell system must first be heated to a temperature above which the water generated by the fuel cell system can no longer freeze, since freezing would create problems. The heating device may comprise a burner to generate the required thermal energy, whereby the burner may, for example, be operated with hydrogen that is also used as fuel for the fuel cell system. The heat generated by the burner may be used to heat a cooling fluid, which circulates in a coolant loop that serves to cool the fuel cell stack during the subsequent operation of the fuel cell stack. The coolant heated by the burner of the heating device circulates in the coolant loop and in this manner transfers the thermal energy to the fuel cell stack to preheat the fuel cell stack.
The above-described pre-heating process must be completed in as short a time period as possible, so that the fuel cell system can be started as quickly as possible, even at low ambient temperatures. During this pre-heating process, a significant amount of heat has to be transferred to the coolant in a short time period. This in turn means that a corresponding air flow has to be supplied to the burner, which can be accomplished only with the help of a powerful fan or a compressor. For this reason, the overall electrical power required prior to the actual start of the fuel cell system is very high, since it is necessary to operate not only the above-mentioned fan or compressor, but also the coolant circulating pump and a control unit that controls the heating process. Since normally the fuel cell stack does not generate any power during the heating process, the required electrical power is generally provided by a correspondingly large battery. This is undesirable for weight- and space-related reasons, in particular for mobile fuel cell applications.